FIG. 1A is a side view of a typical parallel-plate, capacitive, plasma processing chamber 100. FIG. 1B is a top view of a substrate 102 processed in the typical parallel-plate, capacitive, plasma processing chamber 100. The typical processing chamber 100 includes a top electrode 104, a bottom electrode 106 supporting a substrate to be processed 102. The top electrode 104 is typically a showerhead type electrode with multiple inlet ports 109. The multiple inlet ports 109 allow process gases 110 in across the width of the processing chamber 100.
The typical parallel-plate, capacitive plasma reactor 100 is used for processing round planar substrates. Common processes are dielectric etch and other etch processes. Such plasma reactors typically suffer from inherent center-to-edge non-uniformities of neutral species.
The center-to-edge non-uniformities of neutral species arises from the differences in one or more of a flow velocity, an effective gas residence time, and gas chemical composition present at the center of the substrate as compared to the flow velocity, effective gas residence time, and gas chemical composition present at the edge. The gas chemical compositions can be influenced by composition and flow of injected gas mixtures; gas-phase dissociation, exchange and recombination reactions; as well as recombination products and byproducts from surface mediated etch.
By way of example, as the process gases are introduced across the width of the processing chamber the plasma 112 is formed between the top electrode 104 and bottom electrode 106. Plasma byproducts 118 are formed by reactions of plasma radicals and neutrals in the gas phase and/or with the surface of the substrate 102. The plasma byproducts 118 are transported to the sides of the substrate where they may exit the plasma and eventually are removed from the chamber by pumps 108. Plasma byproducts can include products from one or more dissociation reactions (e.g., CF4+e−→CF3+F+e−) and/or one or more ionizations (e.g., CF4+e−→CF3++F−) and/or one or more excitations (e.g., Ar+e−→Ar*+e−) and/or one or more attachments (e.g., CF4+e−→CF3+F−) and/or one or more binary reactions (e.g., CF3+H→CF2+HF).
Plasma byproducts 118 can also include substrate etch byproducts including SiF2, SiF4, CO, CO2, and CN. Etch byproducts can also dissociate and react in the plasma 112 to form other species.
Recombination also occurs during the plasma processing. Recombination is a chemical reaction in which two neutral species combine to form a single molecule, the recombination product 120. Recombination typically occurs when the radicals and neutrals from the plasma 112 interact at surfaces such as the bottom surface of the top electrode 104. The recombination products 120 may be transported off the side of the substrate 102 into pumps 108, similar to the plasma byproducts 118. Plasma recombination products 120 can arise from one or more wall or surface binary reactions (e.g., F+CF→CF2, and/or H+H→H2, and/or O+O→O2, and/or N+N→N2). Plasma-surface interactions can also include deposition of films on the wall or other internal surface of the chamber 100 e.g. CFx radicals may deposit a polymer film.
It should be noted that as shown in FIG. 1A, the plasma byproducts are lost from one side of the substrate 102 and the recombination products 120 are lost from the opposite side of the substrate 102 for clarity purposes only. In actual practice, those skilled in the art would realize that both the recombination products 120 and the plasma byproducts 118 are intermixed and lost from both sides of the substrate 102 to pumps 108.
During plasma processing the concentrations of the chemical species vary from the center to the edge of the substrate 102. These species include recombination products 120, the plasma byproducts 118, as well as unmodified injected gases. Due to these nonuniformities in chemical speciation, as well as other possible non-uniform conditions such as substrate surface temperature, ion flux, ion energy, etc., the effective plasma processing of the substrate, e.g. etching of a target film in a structure, varies from the center to the edge of the substrate 102.
By way of example, the plasma radical species could be most concentrated at the center plasma processing regions 114A and 116A over central portion 102A of the substrate 102. Further, the radicals could be somewhat less concentrated in intermediate plasma processing regions 114B and 116B over intermediate portion 102B of the substrate 102. Further still, the concentrations of the radicals could be even less concentrated in edge plasma processing regions 114C and 116C over the edge portion 102C of the substrate 102.
Thus, in this example, if the local radical density controls the plasma-induced etch rate, the highest etch rate would occur in the center plasma processing regions 114A and 116A over the center portion 102A of substrate 102 as compared to a slightly lower etch rate in the intermediate plasma processing regions 114B and 116B over the intermediate portion 102B of substrate 102 and even lower etch rate in the plasma processing of the edge plasma processing regions 114C and 116C over the edge portion 102C of the substrate. This would result in a center-to-edge nonuniformity of the substrate 102 film thickness after processing. Radial variations in etch and deposition rates are typical problems for commercial plasma processing systems, as applied to round flat substrates such as wafers.
This center-to-edge nonuniformity is exacerbated in small volume product plasma processing chambers that have a very large aspect ratio. For example, a very large aspect ratio is defined as when the width W of the substrate is about four or more times the height H of the plasma processing region. The very large aspect ratio of the plasma processing region limits the effectiveness of gas-phase diffusion for mixing neutral species, and thus tends to worsen the non-uniformity of the plasma byproducts 118 and recombination products 120 in the plasma processing regions 114A-116C.
Although this center-to-edge non-uniformity of neutral species is not the only cause of center-to-edge process non-uniformity, in many dielectric etch applications it is a significant contributor. Specifically, neutral-sensitive processes such as gate or bitline mask open, photoresist (PR) strip over low-k films, highly selective contact/cell, and dual-damascene (DD) via etch may be especially sensitive to these effects. Similar problems may apply in other parallel-plate plasma reactors, besides those used for wafer dielectric etch.
In view of the foregoing, there is a need for improving the center-to-edge chemical species uniformity in plasma etch processes.